ORDERING OF TRANSITION-METAL ATOMS IN MCUFES(2) (M=LI, NA) - ELECTRONIC-STRUCTURE AND MONTE-CARLO SIMULATIONS

Previous X-ray investigations have shown the absence of long-range ord er in the distribution of transition metal atoms on the tetrahedral si tes of the layered MCuFeS(2) (M = Li, Na) compounds. We have analyzed the band structure of these crystals for several distributions of Cu a nd Fe atoms by an extended Huckel Hamiltonian. The electronic structur e study supports that Fe-Fe interactions are energetically more favora ble than Cu-Fe contacts, Effective potential parameters for the intera ction between transition metal atoms were derived from the calculated electronic energies. This potential model was used to investigate the equilibrium (Cu, Fe) distribution as a function of the temperature by Monte Carlo simulations. At the synthesis temperature of NaCuFeS2 (T-f = 1070 K), the (Cu, Fe) distribution does not display long-range orde r, a fact that agrees with X-ray diffraction studies, but is character ized by short-range order favoring the formation of chains and cluster s of Fe atoms.